Semiconductor devices such as logic and memory devices are typically fabricated by a sequence of processing steps applied to a specimen. The various features and multiple structural levels of the semiconductor devices are formed by these processing steps. For example, lithography among others is one semiconductor fabrication process that involves generating a pattern on a semiconductor wafer. Additional examples of semiconductor fabrication processes include, but are not limited to, chemical-mechanical polishing, etch, deposition, and ion implantation. Multiple semiconductor devices may be fabricated on a single semiconductor wafer and then separated into individual semiconductor devices.
Metrology processes are used at various steps during a semiconductor manufacturing process to detect defects on wafers to promote higher yield. Optical metrology techniques offer the potential for high throughput without the risk of sample destruction. A number of optical metrology based techniques including scatterometry and reflectometry implementations and associated analysis algorithms are commonly used to characterize critical dimensions, film thicknesses, composition, overlay and other parameters of nanoscale structures.
In most examples, precise monitoring of a semiconductor manufacturing process is performed by one or more stand-alone metrology tools. However, in some examples, the metrology tool is integrated with the process tool performing the fabrication step under measurement. This is commonly referred to as in-situ monitoring.
In one example, structures subject to a reactive ion etch process are monitored in-situ. In some fabrication steps, the etch process is required to etch completely through an exposed layer and then terminate before substantial etching of a lower layer occurs. Typically, these process steps are controlled by monitoring the spectral signature of the plasma present in the chamber using an emission spectroscopy technique. When the exposed layer is etched through and the etch process begins to react with a lower layer, a distinct change in the spectral signature of the plasma occurs. The change in spectral signature is measured by the emission spectroscopy technique, and the etch process is halted based on the measured change is spectral signature.
In other fabrication steps, the etch process is required to etch partially through an exposed layer to a specified etch depth, and terminate before etching completely through the exposed layer. This type of etch process is commonly referred to as a “blind etch”. Currently, the measurement of etch depth through partially etched layers is based on near-normal incidence spectral reflectometry.
In some examples, the wafer under measurement includes periodic patterns. These patterns exhibit unique reflectivity signatures that can be modeled. Thus, model based spectral reflectometry measurement techniques are suitable for estimating critical dimensions of patterned wafers. Unfortunately, currently available in-situ monitoring tools based on spectral reflectometry lack the precision required to meet future fabrication process requirements.
In many practical examples, the semiconductor wafer under measurement includes homogeneous regions of periodic patterns and also non-homogeneous regions including support circuitry, scribe lines, etc. For example, on a memory wafer, the typical size of the homogeneous region is about 50 microns square surrounded by a non-homogeneous region of a few microns surrounding the homogeneous region. Currently available in-situ monitoring tools illuminate the wafer with a collimated beam that illuminates a large circular area of the wafer, including homogeneous and non-homogeneous regions. Typical illumination spot sizes are ten millimeters in diameter, or larger. The reflected light collected over this large area is mixed and analyzed by a spectrometer. Mixing the reflectivity signals from homogeneous and non-homogeneous regions on the wafer fundamentally limits the performance of the metrology system (i.e., measurement accuracy is limited).
The problem of mixing of reflectivity signals from homogeneous and non-homogeneous regions of the wafer is difficult to solve optically because it is not possible to place illumination and collection optics near the wafer within the reactive plasma chamber. This limits the maximum achievable numerical aperture (NA) and the minimum achievable illumination spot size. Without the ability to optically focus on a small homogeneous region of the wafer with minimum spill-over onto the surrounding non-homogeneous region, it does not appear possible to overcome the limits to measurement accuracy due to mixing of reflected signals.
In summary, ongoing reductions in feature size impose difficult requirements on in-situ spectral reflectometry systems integrated with etch and ion implant process tools. Optical metrology systems must meet high precision and accuracy requirements to enable adequate process control. In this context, the mixing of reflectometry signals from different regions of the wafer under measurement has emerged as a critical, performance limiting issue in the design of in-situ spectral reflectometry systems employed to control etch and ion implant processes. Thus, improved metrology systems and methods to overcome these limitations are desired.